Thermography is an image-forming process including a heating step and hence includes photothermography in which the image-forming process includes image-wise exposure and direct thermal processes in which the image-forming process includes an image-wise heating step. In direct thermal printing a visible image pattern is produced by image-wise heating of a recording material e.g. image signals can be converted into electric pulses and then via a driver circuit selectively transferred to a thermal head, which consists of microscopic heat resistor elements, thereby converting the electrical energy into heat via the Joule effect. This heat brings about image formation in the substantially light-insensitive thermographic material. In thermal heads, only those regions which produce heat higher than a certain value are effective for printing, and the regions capable of generating sufficient heat for the printing spread in proportion to voltage applied to the heating resistors. If, therefore, higher voltage is applied to the heating resistors, the size of the printing dots increases in proportion.
U.S. Pat. No. 5,825,395 discloses a printing system, comprising: a thermal head, color thermal recording paper having a surface with a plurality of different color developing layers disposed thereon, said plurality of different color developing layers corresponding to a plurality of different colors, and means for feeding said color thermal recording paper in a feed direction at a feed pitch; said thermal head producing printed dots in a desired color developing layer over said surface of said color thermal coloring paper by selectively and directly heating said color thermal recording paper; said thermal head comprising an array of a predetermined number of heating elements, each of said heating elements having a length, in said feed direction of said color thermal recording paper, and being controllable to radiate a selected level of thermal energy, said array being operationally disposed with respect to said color thermal recording paper so that said selected level of thermal energy radiated by said each heating element produces one of said dots without damaging said color thermal recording paper; and said length of each heating element having a value of from 2 to 3.5 said feed pitch. The feed pitch is defined in U.S. Pat. No. 5,825,395 as the distance between adjacent image density peaks in the feed direction as can be seen from FIG. 4 and heating element lengths in the feed direction of 260 μm, 310 μm, 360 μm, 460 μm and 560 ∞m are disclosed therein, which according to the specification are longer than those in the case of conventional thermal heads. No information is provided with respect to distance between adjacent heating elements.
EP-A 0 500 334 discloses a thermal recording device for forming an image with a dot matrix by applying a thermal head consisting of heat emitting elements arranged in a single row at a first pitch along a primary scanning direction onto a surface of a thermal recording material and moving said thermal recording material relative to said thermal head in a secondary direction perpendicular to said primary scanning direction, and selectively heating said heat emitting elements for each successive movement of said thermal recording material at a second pitch in said scanning direction, wherein: a ratio of a length of said heating elements of said thermal head in said primary scanning direction to said first pitch is 30 to 70%, and a ratio of a length of each of said heat emitting elements of said thermal head in said secondary scanning direction to said second pitch is 60 to 95%. Embodiments 1, 4 and 7 and Examples 1 and 4 disclose aspect ratios, a/b, where a and b are the lengths of each heat emitting element in the primary and secondary directions, respectively, the primary direction corresponding to the lateral direction of the paper or direction of the row of heat emitting elements as shown in FIG. 2, of 60/25 (=2.4), 60/35 (=1.71), 60/44 (=1.36), 60/53 (=1.13) and 85/44 (=1.93) respectively. FIG. 2 also shows that the pitch in the primary direction, Pa, is the distance between the centre of one heat-emitting element and the centre of the next heat-emitting element in the primary direction. Embodiments 1, 4 and 7 and Examples 1 and 4 disclose b/Pa ratios of 60/63.5 (=0.94), 60/63.5 (=0.94), 60/63.5 (=0.94), 60/63.5 (=0.94) and 85/63.5 (=1.34) respectively. Pb is the dot pitch of the matrix in the secondary direction.
U.S. Pat. No. 5,559,546 discloses a method of perforating a heat sensitive stencil, the method comprising the steps of: bringing the heat sensitive stencil into contact with a thermal head having a plurality of heating resistors arranged in a row; moving the heat sensitive stencil in a sub-scanning direction which is orthogonal to a main-scanning direction in which the plurality of heating resistors are arranged; and perforating the heat sensitive stencil in a dot matrix shape with selectively heated heating resistors of said plurality of heating resistors; the method further comprising the further steps of: making the heat sensitive stencil of substantially only a thermoplastic resin film; and making a width of a space between two adjacent heat resistors of said plurality of heating resistors in the main scanning direction 30% or more of a pitch between the two adjacent heating resistors in the main scanning direction so as to regulate a width of a non-perforated portion of the heat sensitive stencil between two adjacent perforations in the main scanning direction to be 20% or more of the pitch between the two adjacent heating resistors in the main-scanning direction. The ratio of b, the length of the heating resistor in the sub-scanning direction, to the pitch Pa between two adjacent resistors in the main scanning direction in the Example according to Embodiment 1 is 40/63.5 (=0.63) and those of Comparative Examples A and B are both 60/63.5 (=0.94).
JP 58-089385A discloses the obtaining of a constant print density all the time even when thermal heads are exchanged by a method in which the same resistor elements as heating elements are provided to the thermal head having plural resistor heating elements, and on the basis of the resistance values of the elements, applied voltage is controlled. A resistor element 5 made of the same material as that of a heating resistor element 2 is provided to part of a thermal head in which heating resistor elements 2-1˜2-7 are provided on a ceramic base plate 1, and also the resistor element 5 is connected to the terminals Ra and Rb of a constant-voltage regulation circuit through a lead pattern. When connecting a thermal head in which the resistance value of the heating element is lower than that of standard heating element, the base voltage of a transistor Tr2 rises because the resistance value of the resistor element 5 is also lower than standard, and therefore, collector current is increased and the voltage drop of the resistor R1 is increased. By this, the impedance between the collector bases of the transistor Tr1 is increased and output voltage Vout becomes lower than standard. When the resistance value of the heating element 2 is high, the output voltage also becomes higher.
U.S. Pat. No. 4,841,120 discloses a thermal head for recording on a recording medium comprising: a substrate having a thick portion and a thin portion having a first flat surface and a second surface formed on an opposite side of said substrate from said first flat surface, said second surface being adapted to contact said recording medium, a plurality of heat resistor elements formed on said first flat surface of said thin portion of said substrate, wiring circuit means for said heat resistor elements formed on said first flat surface of said substrate and driving means for driving said heat resistor elements formed by said first flat surface of said substrate, said thin portion being made by grinding said second surface of said substrate. FIGS. 7 and 8 illustrate split resistors.
JP 61-086271A discloses the elimination of the need to provide an interpolation line even at the time of high-speed recording, by a construction wherein the first and the second heating dot group are provided on the same substrate, and the two systems of the heating dot group are selectively used in accordance with recording condition, i.e., high resolution recording or high-speed recording. The first and third feeder lines 3, 19 and the second and fourth feeder lines 4,20 are so provided as to clamp respectively both side parts of a heating resistor 2 therebetween, and two heating dots 17, 18 differing in length are provided on the same substrate. In high-resolution recording, the first heating dot 17 is selectively operated for recording by using the first and second feeder lines 3,4 whereas in high-speed recording, the second heating dot is selectively operated for recording by using the third and fourth feeder lines 19,20.
U.S. Pat. No. 5,485,193 discloses a line-type thermal head for half-tone printing which expresses various densities by utilizing printing dots of various sizes, the thermal head having a main scanning axis and comprising: a substrate, a plurality of heating elements arranged on a substrate along the main scanning axis, each of the heating elements including at least one non-rectangular parallelogramatic resistor for generating heat; and means for supplying electric energy, an amount of which corresponds to a size of a printing dot to be recorded, to each of the heating elements to make the resistor generate heat, wherein the resistor has a region which generates sufficient heat for recording the printing dot, and a size of the region is changed in response to the amount of electric energy applied to the resistor so that printing dots having various sizes are produced by each of the resistors, the supply means including lead electrodes connected electrically to one pair of opposite sides of the resistor, each of the lead electrodes having a width not less than a length of one side of the one pair of opposite sides of the resistor; wherein a ratio of the length of one side of the one pair of opposite sides of the resistor to that of one side of another pair of opposite sides of the resistor is not greater than 1.5 and an acute angle formed by two sides of the one and another pair of opposite sides of the resistor is no more than 45° C.
U.S. Pat. No. 5,483,274 discloses a thermal head for a thermal recording apparatus, comprising: means for recording a continuous line of elliptic dots on a recording sheet, said means including a plurality of heat generation resistors arranged one-dimensionally along a given direction, each of the heat generation resistors formed to have a parallelogram shape including-four sides and two diagonal lines and configured such that the four sides of the parallelogram shape have directions crossing the given direction and such that two diagonal lines extending between opposing corners of the parallelogram have directions crossing the given direction; and a plurality of drive electrodes respectively connected to said heat generation resistors; wherein an ink film and the recording sheet for thermal recording, which are stacked on one another, are brought into contact with said heat generation resistors and moved in a direction orthogonal to the given direction along which said heat generation resistors are arranged and, during this movement, ink coated on the ink film is melted by said heat generation resistors to allow and image to be transferred to the recording sheet.
U.S. Pat. No. 4,970,530 discloses a thermal head arranged in a printing device so as to be opposite to a printed object fed in a constant direction, the thermal head comprising: a base member, a plurality of heating resistors arranged in said base member; electrodes disposed in said base member corresponding to said heating resistors; and a slit formed on a surface of said heating resistors and having a shape in which the width of the slit in a feeding direction of said printed object is less than that in an arranging direction of said heating resistors perpendicular to said feeding direction and the width of the slit in the arranging direction of the heating resistors is approximately equal to a half length of a pitch of said resistors in said arranging direction thereof.
Conventional thermal heads, such as used in the thermal head printers manufactured by AGFA-GEVAERT N.V. e.g. DRYSTAR™ 2000, DRYSTAR™ 3000 and DRYSTAR™ 4500, have a ratio of heating element length in the transport direction, L, to the pitch, P, between adjacent heating elements, the so-called aspect ratio L/P, of between 1.5 and 1.80. The thermal head disclosed in EP-A 1 006 000, EP-A 1 006 403 and EP-A 1 006 404 had heating elements with dimensions 85 μm×85 μm i.e. an aspect ratio L/P of 1.0.
There is a need for using a given substantially light-insensitive thermographic material developed for high throughput with a conventional thermal head for printing at lower throughputs without a significant change in image tone and other imaging properties.